This invention relates to a load sensor having strain gauges attached to a load-sensitive element and a temperature-sensitive element with which the effects of temperature variations on the output from the strain gauges can be compensated.
A load sensor for converting a force or a weight into an electrical signal is generally formed by attaching a plurality of (generally four) strain gauges to a load-sensitive element made usually of an aluminum alloy, connecting these strain gauges to a bridge circuit, and providing a temperature-sensitive resistor element for detecting the temperature of the load-sensitive element in order to make corrections on the output from the bridge circuit. FIG. 6 shows an example of such a prior art load sensor comprised of a bridge circuit 120, operational amplifiers 121 and 122, and a differential amplifier circuit 127. The bridge circuit 120 is formed by connecting strain gauges 104, 105, 106 and 107 of a copper-nickel alloy foil attached to a load-sensitive element and its signal output terminals are connected to the non-inversion input terminals of the operational amplifiers 121 and 122 of which the inversion terminals are connected to a temperature-sensitive resistor element 125 for detecting the temperature of the load-sensitive element. The differential amplifier circuit 127 is adapted to receive the outputs from the operational amplifiers 121 and 122. Variations in the load signals from such a load sensor due to the temperature characteristic of the Young's modulus of the aluminum material of the load-sensitive element as well as that of the strain gauges are corrected by adjusting the amplification of the amplifier circuit provided with the temperature-sensitive resistor element 125. In FIG. 6, numeral 126 indicates a precision resistor.
A load sensor of this type allows temperature corrections of its load signals with high accuracy but only for temperature variations within a relatively narrow range including a reference temperature for correction T.sub.0. As shown in FIG. 7, if the temperature variation from this reference temperature T.sub.0 becomes large, the error (indicated by .DELTA.L in FIG. 7) becomes large suddenly. This is because both the temperature characteristic of the output from the bridge circuit and that of the output from the amplifier circuit including the temperature-sensitive resistor element 125 have positive second-order characteristics, or a positive second-order temperature coefficient. Throughout herein, if the temperature-dependence of a physical quantity (such as an output from a circuit) can be approximately written as a polynomial function of temperature as measured from a certain reference temperature and if the coefficient of the second-order term (or first-order term) of this polynomial function is positive (or negative), this physical quantity will be said to have a positive (or negative) second-order (or first-order) temperature coefficient.
In order to overcome this problem of prior art load sensors as described above, the present inventor has earlier attempted to make use of a combination type temperature-sensitive resistor element with a first temperature-sensitive resistor piece not having a positive temperature coefficient and a second temperature-sensitive resistor piece having a positive temperature coefficient, thereby providing a load sensor with a superior temperature characteristic over a wider range of temperature (Japanese Patent Application Tokugan 1-58753, filed Mar. 9, 1989). In using such a load sensor, however, the operator must set the resistance values while adjusting the resistance ratio between the first and second temperature-sensitive elements. In other words, correction of effects due to temperature variations was extremely cumbersome with prior art devices.